Field of the Invention
One disclosed aspect of the embodiments relates to an electrophotographic image forming apparatus.
Description of the Related Art
An image forming apparatus of a tandem system in which color image formation is conducted by transferring toner images, formed in image forming stations of a plurality of colors, in a superimposed manner to an intermediate transfer member has been proposed. In the image forming apparatus of a tandem system, when the toner transferred to the intermediate transfer member in an upstream station passes through a downstream station, a potential step may be caused on a surface of the photoconductor at a transfer portion of the downstream station. It is therefore known that a phenomenon of “ghost” in which the generated potential step appears in the image as a difference in density may occur.
A known example of a method for controlling the ghost is to remove residual charge of the photoconductor by causing a photoconductor surface after transfer to be irradiated with a pre-exposure device having a light source, such as LED.
There is a problem, however, that deterioration of the photoconductor is promoted due to repeated irradiation with light of the surface of the photoconductor by the pre-exposure device for a long period of time, and that image defects generating at the end of life of the photoconductor in a “DC charging system” in which charging is conducted when a DC voltage is applied to a charging roller may occur earlier. It is therefore required to reduce the amount of light to be emitted from the pre-exposure device as much as possible.
In some cases, the pre-exposure device and a guide member used as a path on which light output from the light source passes may be incorporated in a photoconductor unit which is detachable with the photoconductor. When an unused photoconductor unit is attached in this configuration, a distance between the guide member and the photoconductor or a positional relationship between the light source and the guide member may differ for each photoconductor unit. Therefore, when an unused photoconductor unit is attached, the amount of light received by the surface of the photoconductor, emitted from the pre-exposure device, may vary due to an influence of individual difference of the photoconductor unit. It is therefore required to adjust the amount of light emitted from the pre-exposure device to be as small as possible and to be an appropriate amount.
Japanese Patent Laid-Open No. 2009-42738 discloses a method for setting an amount of light for exposure appropriate for controlling occurrence of ghost. In the method, a charging current flowing through a surface of a photoconductor after irradiation of the surface of the photoconductor by the pre-exposure device is detected, and an amount of light for exposure appropriate for controlling occurrence of ghost is set in accordance with the detection result.
In the image forming apparatus, a resistance value of the intermediate transfer member, the transfer roller, and the like may vary due to deterioration of long time use or environmental change, and a potential of the photoconductor after transfer may be changed.
The present inventors have found that, even in a case where the resistance value of the intermediate transfer member, the transfer roller, and the like is varied, and the potential of the photoconductor after transfer is changed, ghost can be controlled if the photoconductor could receive a predetermined constant amount of light from the pre-exposure device.
In the image forming apparatus described above, however, if the amount of light for pre-exposure at the time of image formation is set in accordance with the current flowing in the charging means as disclosed in Japanese Patent Laid-Open No. 2009-42738, a changed amount of the current due to the changed potential of the photoconductor after transfer caused by a change in resistance of the intermediate transfer member, the transfer roller, and the like is contained in the current flowing in the charging means. There is therefore a problem that an amount of light of the pre-exposure device could not be set to be appropriate.